The testing everyone did in the thread Brant linked was primarily looking at the difference between the 60mbs and 90mbs card. For the questions Busted Knuckles posted the answer is yes the card is going to matter (somewhat). The 20mbs and 30mbs cards will limit his performance.

As Brant said, in camera the 7D and 1D IV maxed out at about 50-60 MB/sec, and the only way you really notice this is the cameras ability to recover from a full burst. If you continue to shoot with the button held down after a burst, over time you will get a few less frames because of the slower recovery speed. The difference for the most part is inconsequential for the 5D II and the 7D, and at best very fractional with the 1D IV.

As Brant said, you get a faster transfer speed.

If you want to do video, get the fastest card you can.